Substance use disorder (SUD) is the second most common psychiatric diagnosis in the U.S. population, with 14.6% developing it in their lifetime. Transition-age youth (TAY) from ages 18-24 are especially vulnerable to substance use, misuse, and SUD. This developmental period, termed the age of instability includes tasks such as leaving home; entering college; working for the first time; body and sexuality changes; coalescing with a peer group; and for some, aging out of foster care or state custody, which end at age 18. This period is also prime for experimentation with substance use and associated risks such as driving under the influence, accidents, fighting and violence, HIV, and vulnerability to sexual assault. In this project, we focus on a major strategy, grounding, for helping to de-escalate (reduce) intense negative emotions and impulses. Grounding has long been used in psychiatric hospitals to provide a safe, quick, and powerful way to reduce any intense negative feeling, including impulses to hurt self or others, substance craving, anger, anxiety, etc. Described in detail in the book Seeking Safety (SS), grounding uses three pathways to focus the mind (mental), the body (physical), and the heart (soothing). Grounding provides a way to regulate emotion, which is a major challenge for youth, especially given their hormone changes and emotional intensity. Indeed, substance use is often described as a short-term way to regulate emotion. Grounding is a key module of SS, a widely-implemented, evidence-based SUD treatment model. In this proposal we focus exclusively on grounding as it is an essential skill and widely identified as easy to learn across even the most complex and vulnerable clients. We propose to turn grounding into a smartphone app because grounding should be used in the environment in which the distress is experienced; it capitalizes on TAY use of smartphones; promotes self-regulation; and is a stand- alone skill and sensory-oriented, thus ideal for a mobile app approach. Our specific aims are: (1) to develop a new product, a smartphone Grounding App. This will be an exciting, interactive Android app that uses state-of- the-art gaming technology to engage TAY in grounding, in collaboration with an outstanding company for the technology side of the project. (2) To use an iterative product development methodology based on end-user feedback. We propose various methods to obtain initial input from TAY with SUD before beginning app programming. (3) To conduct a small feasibility study of the app. This cross-sectional pilot will obtain information on end-users experience, usage, and satisfaction with the app, which can inform a phase II study. We will assign 24 TAY with SUD to the grounding app (n=12) versus receiving hard-copy grounding handouts from the SS book (n=12), to evaluate the relative impact of the app. Our team represents an exceptional mix of expertise in SUD, TAY, app and gaming technology innovation, grounding, SBIRs, treatment development, and public-health oriented products based on evidence-based care.